Reuters

SHANGHAI - An emergency descent by an Air China aircraft after oxygen levels dropped in the cabin has been linked to a co-pilot who was smoking an e-cigarette during the flight, state media said on Friday, citing China's aviation regulator.

"In the preliminary investigation, the co-pilot was found to be smoking an e-cigarette," state-owned China News said in a report, citing a news conference by the Civil Aviation Administration of China investigating Tuesday's incident.

"Smoke diffused into the passenger cabin and relevant air conditioning components were wrongly shut off, without notifying the captain, which resulted in insufficient oxygen," it added.

The Air China Boeing 737 aircraft was flying to the Chinese city of Dalian from Hong Kong, when it made an emergency descent to 10,000 feet (3,048 m), during which oxygen masks were deployed, before climbing again to continue on to its destination.